This invention relates to gas handling systems and, in particular, to what is known as a purge block.
In the fabrication of semiconductor devices, and other applications, gases are used which are toxic, flammable, corrosive, explosive, or even some combination of these. While many precautions are taken with the use of such gases, the most difficult situation with which one must cope is the disconnection of the "bottles", i.e. the pressurized metal containers. Supply lines and connecting fittings all contain residual amounts of gas which must be purged before the bottle is disconnected from the system.
To this end, a purge block is provided. To date, a purge block has been simply a fitting at the nexus of a supply line from the bottle, a supply line to the process equipment which uses the gas, a supply line for an inert gas, and a vent or vacuum line. Arrayed about the purge block are three valves, one connected to each of the latter three lines. To disconnect the gas bottle, one typically closes the valve to the process line and the valve on the bottle and then opens the valve to the vacuum line to evacuate the block, then closes the vacuum line valve and opens the valve to the inert gas line, then closes the valve to the inert gas line. After several cycles, the bottle is then disconnected. The number of cycles depends upon the nature of the gas, the volume to be purged, and the absolute pressures in the vacuum and inert gas lines.
While disconnection of the bottle is the most difficult situation, it is not the only time the condition of the gas delivery system must be monitored. The system is monitored continuously, e.g. for bad fittings, regulators, or valves and for human error, all of which usually result in an excess gas flow.
To cope with this, a number of sensors, safety valves and the like have been added, to the point that the purge block and its associated hardware occupy a considerable amount of space in the gas cabinet where the bottles are stored. This is particularly a problem where several bottles, of the same gas or different gases, are stored in the same cabinet. The increase in hardware also increases the chance a mistake may be made and increases the volume to be purged before a bottle can be disconnected. The cost associated with a gas cabinet of the prior art is not insignificant either since the additional components and fittings are quite expensive.
A problem with this proliferation of paraphernalia is the number of connections, particularly those connections the integrity of which is not monitored. While a variety of solutions have been proposed or even implemented, the problem remains that the gas cabinet is a complex, expensive piece of equipment.
Another problem with the systems of the prior art is that the flow sensors in such systems interpret the initial venting or excess purge flow as a system failure. The result is more than the disconcerting sound of an alarm. There is an automatic shutdown which must be reset before the cabinet can be placed back on line. The net result is increased downtime and expense.
One solution to the problem of false alarms is to use an electronic time delay, i.e. the alarm condition is not recognized unless it persists for some predetermined period of time, e.g. three seconds. A problem with this approach is that, if the alarm is real, no corrective action is taken for that same period of time.
Maintenance and/or repair is also a problem since there are so many separate pieces in the cabinet. Joints are not intended to be opened and closed frequently. The individual components can be inspected in the cabinet to some degree, but cannot be fully tested unless removed. The result is that maintenance or repair more closely resembles rebuilding the system.
In view of the foregoing, it is therefore an object of the present invention to provide an improved purge block.
Another object of the present invention is to provide a purge block which incorporates sense means for sensing system failures or human errors around itself or in the process line.
A further object of the present invention is to reduce the volume which must be purged before the bottle of reactant gas is disconnected.
Another object of the present invention is to reduce the space taken up by the purge block and associated sense apparatus in a gas cabinet.
A further object of the present invention is to simplify the gas handling apparatus in a gas cabinet.
Another object of the present invention is to simplify the repair and/or maintenance of a gas cabinet.
A further object of the present invention is to reduce the number of unprotected connections in a gas cabinet.
Another object of the present invention is to eliminate false alarms during purging.
A further object of the present invention is to provide an immediate alarm and shutdown if improper conditions exist.